The present invention relates to an optical beam scanning device including two mirrors for imaging a beam scanned by a rotating polygon mirror on an image plane.
In a scanning optical system using lenses for a post-deflection optical system of an exposure apparatus, there are design restrictions as achromatization for preventing displacement of beam position and imaging position according to variations in wavelength. This is because refractive indexes of lens materials become different according to the difference in wavelength. Contrary, a mirror has an advantage that it has less wavelength dependence, and inventions using mirrors for the scanning optical system have been made. As an example, there is a laser scanning device described in Japanese Patent Application Laid-Open (JP-A) No. 2001-56445.
In this laser scanning device, two mirrors are used in a post-deflection optical system as shown in FIGS. 1 and 2, and powers of the two mirrors in the main scanning direction are positive and positive, respectively.
Accordingly, a principal point by the two mirrors is located between the first mirror and the second mirror, and the distance from the last imaging element to the image plane can not be taken larger than the f value of an fθ mirror.
Further, JP-A Nos. 2001-13441 and 2001-13442 discloses a device using one mirror, and, in this case, a principal point is located on the mirror surface. Accordingly, the distance from the last mirror surface to the image plane becomes nearly equal to the value of the f value of the fθ mirror.
Further, there is a device using a pair of plastic lenses as one employing optical elements using inexpensive molded components, and there is a restriction that large powers can not be provided to the lenses themselves in order to suppress the influences of temperature and humidity to the refractive indexes and shapes. On this account, it has been necessary to make an effective field angle smaller and raise image frequencies in order to secure the distance between the last imaging element and the image plane. If aluminum evaporation is performed on the molded component for use as a mirror, the influences of temperature and humidity to the refractive indexes can be removed. In the past, there has been a device in which the first mirror is formed in a concave shape having a positive power, however, the image side principal point becomes nearer the rotating polygon mirror side than the second mirror, and the distance between the last imaging element and the image plane becomes shorter.
In order not to be affected by irregularities in faces of the rotating polygon mirror (amounts of variation in the case where the distance from the rotational center to the reflection surface differ from face to face), it is considered that a beam nearly paralleled to the main scanning direction enters the rotating polygon mirror (see FIG. 1). In this case, the f value of fθ characteristic becomes nearly equal to the combined focal length in the imaging system. Accordingly, the location at the combined focal length from the image plane side principal point becomes an image plane.
As one method for securing the distance between the last imaging element and the image plane, there is a method of making the f value larger. However, this makes the θ value smaller when scanning the same width. In this case, there are following two drawbacks.    (1) When the θ value becomes smaller, the image frequency as a frequency of laser ON/OFF corresponding to one pixel is raised.
The beam position is designed so that the relationship h=fθ is satisfied. Assuming that the size of one pixel is Δh and the deflection angle when one pixel is formed is Δθ, Δh=fΔθ holds. The equation is expressed by Δh=fωΔT where the deflection angular velocity of the beam is ω and the time for forming one pixel is ΔT. Accordingly, the image frequency is expressed by 1/ΔT=fω/Δh. This shows that the image frequency is proportional to f, and, as f is made larger, the image frequency is also raised.    (2) Since, as the smaller the θ value, the distance from the reflection surface of the rotating polygon mirror to the last surface of the imaging element also becomes larger in proportion to f, the entire optical path length becomes longer and the unit becomes upsized.
As another method, there is a method of making the image side principal point in the main scanning direction nearer the image plane side than the last imaging element.
The case where two optical elements having powers φ1 and φ2 are located at distance d2 is considered using paraxial ray theory.
The combined power isφt=φ1+φ2−d2×φ1×φ2  (1)
The object point side principal point position is expressed from the first optical element by:Δ1=d2(φ2/φt)  (2)
The image plane side principal point position is expressed from the second optical element by:Δ2=−d2(φ1/φt)  (3)where Δ1 and Δ2 are positive at the image plane side.
Here, in the main scanning direction, it is necessary to set the combined power φ1 positive so that a nearly parallel beam enters the optical element and is imaged on the image plane.
To make the image plane side principal point position nearer the image plane side than the second optical element, the power allocation may be made so that Δ2>0 holds.
Since the distance between elements is positive, d2>0 holds. As described above, since the combined power is positive, φt>0 holds. From the above conditions and (Eq3), if φ1<0, Δ2>0 can be realized.
Furthermore, to make the combined power positive, the following condition is required. First, rewrite (Eq1) and the condition is expressed by:φt=φ1+φ2(1−d2×φ1)Since d2>0 and φ1<0, and (1−d2×φ1)>0 and φ1<0, it is necessary that at least φ2>0 so that φt>0 holds. To be precise, it is necessary that φ2>−[φ1/(1−d2×φ1)] holds.
Therefore, in an optical system including a pair of optical elements, when the optical element at the object point side is formed by an optical element having a negative power and the optical element at the image plane side is formed by an optical element having a positive power, the image side principal point can be made nearer the image plane side than the image plane side optical element (see FIG. 3).
For the purpose of achromatization of axial aberration, the configuration with a pair of lenses in which the power of the lens at the deflector side is made negative and the material of the lens at the deflector side is made to have high refractive index and high dispersion property has been developed. The purpose is in achromatization as a problem specific to lenses. On the other hand, there is no change in power due to wavelength in a mirror and there is no problem of achromatization specific to lenses. Accordingly, for mirrors, examination on power allocation as above lenses has not been made.